Spin coating has been used for decades as method to coat a flat surface with a thin layer of polymer, photo resist, or other compound. Spin coating is typically carried out by depositing a solvent solution, polymer solution, or other liquid material onto a flat substrate. The substrate is then rotated at an angular velocity sufficient to create a centrifugal force that causes the solution to flow outward toward the edge of the substrate, thereby coating the entire surface of the substrate. Excess solution is ejected from the edge of the substrate, and the remaining solution thins and hardens as the solvent evaporates, leaving a thin polymer film.
Such spin coating is a routine step in photolithography used in semiconductor device manufacturing. In an example photolithographic process, a resist spin coating step is executed to form a uniform resist film on a semiconductor wafer. Next, an exposure process typically involves exposing the resist film to light or other radiation through a mask that creates a latent line pattern. Finally, a developing step involves developing the resist-coated wafer after an exposure process to reveal the line pattern. Such a series of processing stages is typically carried out in a coating-developing system.
In a typical spin coating process, a semiconductor wafer or other substrate is rotated together with a spin chuck by a rotational drive mechanism. The wafer can be vacuum-fixed on the spin chuck or otherwise held. A resist nozzle positioned above the semiconductor wafer drops a resist solution onto the center of the wafer surface. The dropped resist solution spreads radially outward toward the circumference of the semiconductor wafer by centrifugal force as the wafer spins. Although spreading the resist across the entire wafer surface happens relatively quickly, the semiconductor wafer is continuously rotated (usually at a decreased rotational speed) for a period of time to spin off and dry the resist solution spread over the wafer surface. Such spin coating has been used extensively in the semiconductor industry, primarily to form a thin, uniform layer of photoresist polymer on the surface of a wafer as a preparatory step for further wafer processing.